This application relates to an improved fuel pumping system wherein a servo gear and a main gear act in cooperation in distinct manners dependent on varying system conditions.
Gas turbine engines are known, and typically include a compressor compressing air and delivering it in a combustion chamber. The compressed air is mixed with fuel in the combustion chamber, combusted, and the products of combustion pass downstream over turbine rotors, driving the rotors to create power.
There are many distinct features involved in a gas turbine engine. As one example only, the compressor may be provided with variable vanes which are actuated to change an angle of incident dependent on system conditions. Actuators for changing the angle of incident of the vanes are provided with hydraulic fluid from a servo gear pump.
Further, a main gear pump is utilized to deliver fuel into the combustion chamber. It is known in gas turbine engines that fuel can be used as the hydraulic fluid in the accessory actuators as mentioned above.
In a known system, the operation of the servo gear pump requires especially high flow at early operation conditions. However, under more steady state operation, the servo gear pump does not necessarily need to move as much fluid to the accessory actuators.
On the other hand, the main gear pump for supplying fuel to the combustion chamber does not necessarily need as much flow at the start of operation. It does increase its flow needs as the engine continues to operate.
Thus, it is known to provide a “minimum pressure valve” on a line downstream of the servo gear pump. After the servo gear pump has built up its pressure, the valve opens and then fuel from the servo gear pump can supplement the fuel from the main gear pump being delivered into the combustion chamber.
As the number, and complexity of the accessories associated with a gas turbine engine increases, the size of the servo gear pump necessary to supply adequate fuel to each of the actuators is also increasing.